Migrate custom 'About Tor Browser' dialog from torbutton to tor-browser branding

As part of branding update change in legacy/trac#25702 (moved), the Tor Browser icon referenced in the About Tor Browser XUL dialog was changed to one packaged in tor-browser (since torbutton does not know whether it's stable, alpha or nightly).

The entire dialog should be moved to tor-browser rather than having cross-referenced content. As part of this, also change the 'Tor Browser' text from an image to just text.